Support for GridFS append mode

This change add the ability to open a gridfs file in append mode and add data to it, without having to pull it from the db and put it back once modified.

Currently the only way to append some data to a file is to dump the entire file to a temporary file, append the data and then put it again into the gridfs, a very expensive way to add some bytes to a file.

Hope it will be useful to everyone.

best regards.